Rat alpha-lactalbumin (alpha-LA) mRNA has been purified from the total RNA of 4-day lactating mammary gland by poly(U)-sepharose chromatography and by fractionation of the poly(A)plus RNA on sucrose gradients. Alpha-lactalbumin was homogenous as judged by electrophoresis in (a) urea-agarose gel and (b) 1.5% agarose gel under glyoxal-denaturation conditions, which gave a molecular weight of 210,000. The chymotryptic fingerprints of the protein synthesized with this mRNA, in a translational system from wheat germ, where similar to those of purified rat alpha-LA. In rat milk or lactating gland, the content of 42K casein is 6 times times greater than 29K casein and 10 fold greater than alpha-LA. However, alpha-LA mRNA (210,000 M.W.) and two casein mRNAs (460,000 M.W. and 390,000 M.W.), coding for 42K and 29K caseins, were present in equal proportions when measured with cDNA probes in the total RNA of 4-5 day lactating gland. Moreover alpha-LA and total casein were synthesized in a ratio of 1:2.5 in a wheat germ translational system using the total RNA of the lactating gland. The results suggest post-transcription controls in the synthesis of these proteins.